Oil-based inks have long been used widely with ball-point pens. The oil-based inks, however, have a drawback of corroding metallic balls, and to overcome this corrosion problem, cemented carbide balls using tungsten carbide have been developed.
As water-based inks have begun to be used for the ball-point pens in recent years, even the cemented carbide balls sometimes fail to offer sufficient corrosion resistance. To deal with this situation, chemically inert ceramics balls having high corrosion resistance against water-based inks have been developed. These ceramics balls use various kinds of ceramics material, such as alumina ceramics, silicon carbide ceramics and zirconia ceramics.
Although the ceramics materials mentioned above pose no problem in terms of corrosion resistance, they have the following drawbacks and thus are not satisfactory.
The alumina ceramics balls are currently not used very much because they easily chip or crack due to flaws in their crystal structure when they are polished into balls and it is difficult to produce satisfactory ball surfaces.
The silicon carbide ceramics balls, though there is no problem with mechanical strength, have a drawback that the dense, hard sintered body with a surface Vickers hardness (Hv) of 2000 or higher makes polishing difficult. For satisfactory polishing, however, the preferred hardness should be not more than Hv 2000. Because the perfectly sintered ideal silicon carbide has Hv 2400-2600, the sintering process must be stopped halfway to make the hardness less than Hv 2000. This may cause variations in the hardness of the sintered silicon carbide or make void control on the ball surface difficult.
To improve affinity between balls and ink, it is common practice with cemented carbide balls that their mirror-finished surfaces are formed with fine indentations. Since the silicon carbide ceramics balls are very hard, appropriate indentations cannot be formed in their surfaces and there is no alternative but to use the mirror-finished balls. Therefore, the affinity between the ball surface and ink is insufficient, and this easily causes line break and line splitting while writing.
The zirconia ceramics balls are lower in hardness than silicon carbide and thus polishing is easier, but their drawback is that scratches are easily formed in the ball surfaces. If a ball with scratches is used, a ball seat supporting the ball in the tip of a ball-point pen will wear significantly, with the result that the ball position is retracted from the normal one (a ball sink phenomenon), and the writing feel is badly degraded.
The mirror surface of the zirconia ceramics ball has very few voids with its state similar to glass, so its wettability with water-based ink or adhesion and holding capability with oil-based ink is not enough, causing line break and line splitting during writing in some cases. Hence, it is necessary to check that the kind of ink used is an appropriate one.